1. Field of the Invention
This invention relates to a method of manufacturing a molded polyurethane product with a permanent set that allows the product to retain dimensional stability during temperature fluctuations. More specifically, the invention relates to the manufacture of a polyurethane wear ring for use in jet pumps or the like that can withstand the temperature cycles associated with such uses.
2. Description of the Prior Art
It is generally known in the art that viscous liquid urethane polymers derived from isocyanate (-NCO) terminated polyesters or polyethers (with or without some branching or chain extension) can be mixed with a cross-linking curing agent (e.g., diamines or the like) and poured into a heated mold such as to cure the cast polyurethane part. It is also generally known that such urethane rubber cast parts can then be removed from the mold and subjected to further heating and post cure cross-linking. Liquid cast urethane rubbers are noted for their high tear strength and abrasion resistance particularly relative to natural rubbers. Extraordinary hysteresis under dynamic stressing can be rationalized as the cause of the desirable wear and abrasion resistance, however, the lack of dimensional stability under temperature cycling (i.e., non-reproducible thermal hysteresis) severely limit the pragmatic use of liquid cast polyurethane components for at least two reasons. First, since the liquid casting, curing and post cure annealing are generally performed at elevated temperatures, the resulting external dimensions of the cast polyurethane part is difficult to predict and control. Thus in the past when high precision in a given dimension is critical, the part would generally have to be machined to tolerance rather than directly cast to the desired dimension. And secondly, even if the cast polyurethane part were machined to proper tolerance, the end application of the resulting urethane rubber would have to be confined to relatively narrow range of temperature for thermal excursion beyond this range would result in a change in the dimension of the urethane rubber part.
For example, it has been suggested in the past that the metal or brass wear ring surrounding the outer perimeter of the impeller of a modern jet pump be replaced with rubber material, however, when attempting to do this, two very specific deficiencies or problems occur. First, in such an application and particularly when the jet pump is intended for high performance drag boat racing, the rubber wear ring must conform to dimensional tolerances measured in terms of a few thousandths of an inch. Prior to the present invention, these close tolerances required the liquid cast polyurethane part to be machined after molding. And second, even after being machined to fit the jet pump housing, the liquid cast polyurethane rubber wear ring, prior to the present invention, would experience dimensional instability, particularly if the part was exposed to the natural temperature variations associated with the summer and winter seasons. The polyurethane wear ring would not return to the original dimensions after experiencing cool weather which in turn would result in the ring becoming loose in the jet pump housing and/or failure of the hydraulic sealing on the outer perimeter of the impeller and associated loss of efficiency of the pump.
In view of the prior art and in particular, the problems associated with precision wear surface applications such as found in the marine jet pump and the like, a need still exists for a method of manufacturing a liquid cast urethane rubber part that can be molded to exacting tolerances, thus alleviating any post molding machining steps and simultaneously exhibits improved dimensional stability over a broader range of operating temperatures. The present invention provides such a process and resulting cast polyurethane part.